(S)-tert-butyl (1-((4,6-dimethyl-5-oxo-5,6-dihydrobenzo[c][2,7]naphthyridin-8-yl)oxy)-4-methylpentan-2-yl)carbamate (300 mg, 0.683 mmol) was taken up in AcOH (15 mL) and treated with periodic acid (156 mg, 0.683 mmol) and H2SO4 (0.018 mL, 0.341 mmol). The reaction mixture was heated to 80° C. for 20 min after which time iodine (104 mg, 0.410 mmol) was added and heating was continued for an additional 3 h. After cooling, the volatiles were evaporated. The residue was taken up in ethyl acetate (25 mL) and organic layer was washed with saturated aqueous NaHCO3 (10 mL), then brine (10 mL), dried with Na2SO4, filtered and concentrated under reduced pressure to afford (S)-2-(1-(4,6-dimethyl-5-oxo-5,6-dihydrobenzo[c][2,7]naphthyridin-8-yloxy)-4-methylpentan-2-yl)isoindoline-1,3-dione (300 mg, 0.61 mmol, 90% crude yield) as a brown solid. This was taken to the next step without further purification. LC/MS (ESI) m/e 466.1 [(M+H)+, calcd for C20H25IN3O2 466.1]; LC/MS retention time (method F): tR=0.58 min.
